1. Field of the Invention
The present invention relates generally to tape automated bonding fabrication of semiconductor devices, and more specifically to an improved tape including an arrangement of gold and tin on the connector leads and a method of making and using same.
2. Description of the Prior Art
Tape automated bonding is a method for simultaneously connecting a plurality of bonding pads on a semiconductor device to external circuitry. The method employs a continuous metalized tape having individual frames defining metal leads which are arranged in a pattern so that the inner ends of the leads may be bonded to the bonding pads on the device while the outer ends of the leads may be joined to a conventional integrated circuit package or left free to be otherwise connected to external circuitry.
A semiconductor device generally consists of a plurality of circuit elements, including diodes, transistors, resistors and the like, centrally located in an active region on the semiconductor die. Around the periphery of the active region are a plurality of bonding pads appropriately connected to the circuit elements. External connection is made to the circuit elements via the bonding pads. In making external connections to the bonding pads, it is common in TAB for a "bump" to be formed on the bonding pads or tape to provide solder material.
In the prior art bonding to the bumps on bonding pads of a semiconductor die is accomplished by positioning the lead in contact with the bump and applying heat and pressure to affix the lead in place. Metalization for the solder is supplied by the bump, lead, or both. In tape automated bonding, the lead is placed proximate to, or in contact with, the bump by means of aligning the lead-carrying frame relative to the semiconductor die.
Various combinations of metals for the leads and bumps have been tried to effect improved bonds. For example, in the "DELTA" process (DEnse Lead TApe) a copper-based tin-plated lead is bonded to an all gold bump. The lead is brought in contact with the bump at an applied force of 2-3 g/mil.sup.2 and the interface is brought up to a temperature of 300 to 350 degrees Centigrade to form the bond. The gold and tin are in a particular ratio by weight (i.e., 80% gold to 20% tin) such that in the region of bonding the primary gold-tin eutectic is formed.
One advantage of this process over for example, gold-to-gold thermo-compression bonding is that reduced applied force and heat are required. The gold-to-gold bond requires 15-20 gm/mil.sup.2 applied force and local heating to 500 degrees Centigrade or more in order to form the bond. This high temperature and pressure often has a detrimental effect on circuit components, and can result in malfunctioning or nonfunctioning componentry. Utilizing tin and gold such that the primary gold-tin eutectic is formed means that the force and temperature required to form the bond are significantly reduced. The reduced temperature and pressure results in higher yields of acceptably performing components.
One problem, however, with the use of tin in, for example, a typical eutectic process, is that tin has a tendency to form "whiskers." Whiskers are essentially metal growths that extend from a tin surface. They grow after deposition of tin on lead tape and after bonding where pure tin remains on the lead tape. The exact cause of the formation of tin whiskers is not known. Observation of the whiskers has shown that there are several types of whiskers, including needle, tubular, curl, etc. Their size can range up to five microns in diameter and up to one hundred microns in length. The whiskers can cause shorting and noise in pin tests. In short, whisker formation can have a seriously detrimental effect on device performance.
It is also known that tin whiskers form only on pure tin. They do not form on the gold-tin primary eutectic. Thus any pure tin on the lead or bump remaining after bonding is at risk of having whiskers form thereon. It is therefore desirable to consume the entire quantity of tin in formation of the gold-tin eutectic.